(1) Field of the Invention
The present invention relates to improved Streptococcus lactis containing two plasmids which together encode for mucoidness. Further, the present invention relates to a method wherein plasmid DNA known to encode for mucoidness is used as a probe to determine whether or not the Streptococcus lactis encode for mucoidness.
(2) Prior Art
The occurrence of lactic streptococci that produce a mucoid, ropy texture in milk is well documented (Hammer, B. W., Iowa Agr. Expt. Sta. Research Bul. 74:260-270 (1923)). Such ropy lactic streptococci are used in Scandinavian fermented milks called taette (Foster, E. M., et al., Dairy Microbiology, p. 14-15, 48 and 332, (1957); Rasic, J. L. et al., Yoghurt-Scientific grounds, technology, manufacture and preparations, p. 194 (1978)), Swedish lang mjolk (Bottazzi, V., Biotechnology, Vol. 5, p. 328, 345-346 (1983); Macura, D., et al., J. Dairy Sci. 67:735-744 (1984)) and Finnish villii (Saxelin, M., et al., Canadian J. Microbiol. 25:1182-1187 (1979)). Forsen, R., Finnish J. Dairy Sci. 26:1 (1966) isolated mucoid strains of all three lactic streptococci, namely, Streptococcus cremoris, Streptococcus lactis and Streptococcus lactis subsp. diacetylactis, from Finnish villii.
The instability of mucoid characteristic in lactic streptococci has been observed by several investigators (Foster, E. M., et al., Dairy Microbiology. p. 14-15, 48 and 332 (1957); Hammer, B. W., Iowa Agr. Expt. Sta. Research Bul. 74: 260-270 (1923); and Macura, D., et al. J. Dairy Sci. 67: 735-744 (1984)). Foster et al reported that mucoid lactic streptococci gained or lost the slime-producing property "capriciously". Macura and Townsley found that ropy lactic streptococci lost the mucoid property after 10 or 12 serial transfers; some strains became non-mucoid even after six transfers. Brooker (Brooker, B. E., J. Dairy Research 43:283-290 (1976)) working with a pure milk culture of a ropy S. cremoris strain observed considerable variations in the proportion of cells producing extracellular capsular material. Traditionally, in the production of Scandinavian ropy milks, low temperature incubation between 13.degree. C. to 18.degree. C. is preferred, because incubation at temperatures higher than 27.degree. C. to 30.degree. C. resulted in considerable reduction or loss of desirable high viscosity and mucoidness (Bottazzi, V., Other Fermented Dairy Products. p. 328, 345-346. In: G. Reed (ed.), Biotechnology-Vol. 5, Food and Feed Production with Microorganisms. Verlag Chemie, Weinheim, Federal Republic of Germany (1983); and Macura, D., et al. J. Dairy Sci. 67: 735-744(1984)). It had been suggested by early prior art that the mucoidness might protect the lactic Streptococcus against bacteriophage; however, this was shown to be wrong. Sozziet al., Milchwissenschaft 33, 349-352 (1978).
The association of several metabolic functions in lactic streptococci with plasmid DNA is now well recognized (McKay, L. L., J. Microbiol. 49:259-274 (1983)). On the basis of the observed instability of ropy characteristic in lactic streptococci, Macura and Townsley (Macura, D., et al., J. Dairy Sci. 67:735-744 (1984)) and McKay suggested that plasmid DNA may be involved in the expression of mucoid phenotype (Muc.sup.+).
A problem in the prior art is to be able to detect and produce phage resistant strains of Streptococcus which are members of group N, particularly Streptococcus lactis. It would be highly desirable to be able to impart phage resistance to strains of Streptococcus which are phage sensitive since these bacteria are very important in commercial fermentations for producing fermented milk products. McKay et al., Applied Environmental Microbiology, 47; 68-74 (1984) describes limited phage resistance which is plasmid associated. Klaenhammer, T. R., Advances in Applied Microbiology 30, 1-29 (1984) at page 22 discusses plasmid encoded phage resistance. Phage resistance has not been associated with a 18.5 Mdal plasmid in Streptococcus cremoris encoding for mucoidness. Further, Streptococcus cremoris NRRL-B-15995 was obtained as a single colony isolated from a phage resistant strain but is a slow acid producer and thus is not a useful strain for milk fermentations.